The present invention relates to a procedure for the measurement of the impedance of a patient.
An impedance method is used to measure a patient's respiration and blood circulation, such as cardiac stroke volume, cardiac output, cardiac contractility indices, thoracic fluid content, deep vein thrombosis, peripheral blood flow, and arterial occlusive disease.
Impedance respiration measurement, i.e. respiration measurement by the aid of a variable impedance, is based on the measurement of changes in electric conductivity in the chest. Electric conductivity decreases during inhalation as air flows into the lungs, air having a low electric conductivity, and increases during exhalation as air is discharged from the lungs.
The measurement is generally performed using a highfrequency, constant nominal value current as measuring current, so that the impedance of the chest can be calculated from the voltage set up across it. In this case, the chest impedance is mainly resistive, with a reactive component of only about 15%. Measured with a 100 kHz signal, the resistance of the chest generally has a magnitude of about 20-60 ohms. Respiration only causes a change of approximately 0.1-1 ohm in the resistance, because most the measuring current does not flow through the highly resistive lung tissue but through the muscles and the back.
In addition, the measurement is disturbed by the stray capacitances of the cables and amplifier, although these only have a minor significance because the voltage generated across them is in a different phase relative to the voltage generated by the resistive component of the impedance of the chest.
A high-frequency measuring current is used because the impedance of the electrodes decreases with increasing frequency. The skin-electrode impedance decreases to one hundredth when the measurement signal is increased from a low frequency to 100 kHz. In practice, the upper limit for the measuring signal frequency is about 100 kHz, because at higher signal frequencies too much of the measuring current begins to flow through stray capacitances. The measuring frequencies used are generally in the range of 10-100 kHz.
In impedance measurements previously known, two or more electrodes are used. When the measurement is performed using two electrodes, the measuring current has to be supplied via the same electrodes as are used for the measurement itself. This results in a measurement error, because the current density is larger in the vicinity of the current supply electrodes, and the impedance changes occurring in the tissue near the current supply electrodes appear larger in relation to those occurring in the rest of the tissue. Moreover, the non-linearity of the current density in the tissue causes error in the measurement result.
In practical monitoring, however, the biggest drawback with two-electrode measurement is that the impedances of the electrodes and measuring cables are summed with the chest impedance, with the result that the small change in chest impedance caused by respiration easily gets lost in the total impedance.
To correct the measurement errors referred to above, measuring procedures involving four or more electrodes have been developed. In a four-electrode procedure, the current is supplied to two outer electrodes and the measurement is performed with two inner electrodes. When the measuring amplifier has a high input impedance, the current flowing through the measuring electrodes, which causes the measurement error, is small. Besides, the impedances of the current supply cables and electrodes are not summed with the impedance to be measured. In addition, the measurement area can be located at a larger distance from the current supply electrodes, in which case the magnitude of the current density in the tissue in the area between the measuring electrodes is roughly constant. Moreover, the impedance changes in the area between the measuring electrodes appear larger in relation to the changes in other areas, making it possible to define the measurement area more precisely. In addition, the measurement area extends deeper into the tissue.
By using guard electrodes in addition, the error caused by the non-linearity of the current density can be reduced. By further increasing the number of electrodes used, the measurement area can be focused better and it is even possible to compute an impedance tomogram by means of a computer.
Since impedance respiration measurement in patient monitoring is generally performed simultaneously with ECG measurement using the ECG cable as the measuring cable, two-point measurement is generally used, because the monitors usually have three-wire ECG cables. This means that the above-described problems associated with two-point measurement cause disturbances in the measurement.